We use it, and LOVE it. I don't think there's a better value out there for math.

My child is in 1st grade. I started homeschooling her in November. The public school used Saxon Math, which I do not care for at all.

Mammoth Math is awesome. I can't say enough about it, at least using it for a 1st grader. Since we started her halfway through the year, I bought the grade 1B curriculum which is half a year for $15. I liked it SO much and she learned SO much more using this curriculum, that I went back and bought the 1A curriculum so that she could relearn basic addition with the benefit of Maria Miller's approach.

Maria Miller believes in teaching math facts with pattern and structure, as opposed to the utterly random memorization my child learned, or should I say failed to learn in public school. While my child did learn how to count up and understood what addition was, she had no comprehension of how to internalize math facts beyond memorization. And she was inept at memorization the way she was taught at school.

I do not completely dismiss the importance of some memorization, but if you memorize doubles, study a few fact families by sum in order to see the patterns, and learn how to skip count by 2's, 5's, and 10's, you can basically answer any math fact with very rapid critical thinking (and this is what many competent adults do as well). Math Mammoth really works on that.

She also breaks up the basics with units on geometry, time-telling, measurement, graphs and charts, etc. There was ONE time when I thought a lesson was inappropriate for a 1st grader (a geometry lesson dealing with spacial reasoning that was beyond the average 1st grade capacity). I e-mailed Maria Miller with feedback and within 3 days she had revamped the curriculum and sent me a replacement lesson.

Again I can't speak for all the grade levels, but so far I think the curriculum is fantastic and you can't beat the price. I have a couple entries in my blog where I compare Math Mammoth to Saxon if you are interested as well (link is on my profile.)